The present invention relates to an earthquake isolation floor and more particularly to an earthquake isolation floor particularly adapted for keeping machinery and tools, etc. to be mounted thereon free from vibration such as caused when they are subjected to an earthquake, or the like.
For instance, electronic computers, emergency dynamos, dangerous objects (e.g. dynamite, chemicals, etc.), etc. should continue to operate or be stored in safety even during an earthquake. From this point of view there is considerable attention being paid to anti-earthquake or anti-vibration means for support frames to mount machinery and tools, etc. and for places for storing dangerous objects.
Countermeasures against vibration can be divided into two kinds at present:
(1) Such equipment as atomic energy plants, spherical tanks, electrical substations, etc. are originally designed against earthquakes by examining them structurally so that they will have sufficient strength against seismic input, and increasing their strength. PA1 (2) Machinery and tools, dangerous objects, etc. are secured on a floor which is mounted on springs such that the springs absorb the seismic input when such occurs so that the machinery and tools, etc. can be kept free from danger, such a floor being referred to as a so-called "earthquake isolation floor". PA1 (1) It is difficult to establish the minimum acceleration at which the latch means is released; PA1 (2) It is difficult to reset the latch means after the floor has been once released; PA1 (3) Since no lock means in the horizontal direction is provided, it is difficult to establish a minimum acceleration at which the vibration free effects are revealed; PA1 (4) It is difficult to restore the floor after it has once moved in the horizontal direction; PA1 (5) There is a danger of rocking on the vertical springs; and PA1 (6) Since the transverse rigidity of the vertical springs cannot be ignored with regard to the horizontal springs, the establishment of the horizontal springs and an estimate of their effectiveness, are made difficult.
The present invention relates in particular to the latter, i.e. the "earthquake isolation floor".
Hitherto earthquake isolation floors have comprised a sliding plate, a support frame slidably mounted on the plate with low friction elements being interposed therebetween, a number of springs disposed horizontally between the support frame and a foundation on which the sliding plate is laid, a floor mounted on the support frame through springs disposed vertically, a number of dampers disposed vertically between the support frame and the floor, and a latch means to secure the vertical springs during normal periods. In this earthquake isolation floor the vertically arranged springs are constituted such that they are brought into operation only after the latch means is released due to the onset of a seismic input greater than a predetermined value.
However, this hitherto known earthquake isolation floor has the following problems:
The present inventors have already proposed a new invention entitled "Earthquake Isolation Floor" for the purpose of conquerring such difficulties in the conventional earthquake isolation floors as discussed above. In this connection reference should be made to Japanese Patent Application No. 145419/1979, assigned to Mitsubishi Steel Co., Ltd., corresponding to U.S. Ser. No. 199,980 filed on Oct. 23, 1980. That is, this proposed invention is characterized in that in order to attain its objects a support frame on which machinery and tools, etc. to be prevented from vibration are secured is mounted on a fundamental frame by means of sliding faces, secured to the bottom of the support frame and comprising a material having a low coefficient of friction, and disposed between the confronting ends or peripheries of the support frame and the fundamental frame are horizontal coil springs, each in a precompressed state, and dampers, a cover member being fixedly secured to the fundamental frame so as to surround the support frame with a gap being left therebetween, whereby the horizontal springs as well as the dampers are received within the space formed around the support frame by the cover member, the cover member being adapted to act as a rocking prevention means in association with the support frame and at the same time as a dust protection cover for the sliding surfaces of the support frame.
Although it has been confirmed that this proposed earthquake isolation floor can attain its proposed objects, in order to increase its vibration free effects it is necessary to make the spring constant small, but, in the case of a compression spring as in this invention, since it is prone to easy buckling when the spring constant is small and the spring buckles easily when it is displaced in a direction oblique to its axis, there arises the problem that its vibration free effects cannot be made great.